This invention relates in general to monitoring the loading of an ion-exchange filter by measurement of electrical conductivity.
An ion-exchange filter used for example in a water softener apparatus, is exposed to raw water after regeneration so that the ions stored in a sodium phase, are exchanged for calcium ions in the raw water. The ion concentrations of the sodium and the calcium undergo change until system equilibrium of the ion concentration is established between the exchange phase and the liquid phase. Thus, an ion concentration profile of the calcium is developed in the exchanger which migrates in the flow direction through the exchanger to leave a calcium concentration in the exchange phase which is at equilibrium with the concentration in the aqueous solution being treated, such as raw water. In the aqueous solution, a concentration profile of the calcium is also established, determined by the concentration profile of the exchanger phase.
It is already known in the prior art that the termination of end point loading in an ion-exchange filter may be achieved by measuring in two or more phases perpendicular to the flow direction, the electrical resistances of the filter contents, consisting of the aqueous solution and the exchange phase such as a resin filter medium. From such measurements, the differences in resistance is determined to locate the loading front. Such determined resistance differentials for the filter contents, are utilized to generate a signal for optical and/or acoustic readout and/or to initiate regeneration of the filter medium. According to one prior art proposal, a reference resistance is measured by a pair of reference electrodes and the resistance being monitored is measured by a pair of measuring electrodes. The two pairs of electrodes are interconnected with two additional resistances to form a bridge circuit. After regeneration is completed, the reference resistances and the measurement resistances are the same, so that the bridge circuit is balanced. If the concentration profile of the calcium migrates in the flow direction through the ion-exchanger and reaches the measuring electrode pair, the measurement resistance increases and the bridge circuit is unbalanced. Connected to the output of the bridge circuit is a comparator having one input to which an AC voltage reduced by the measurement resistance is applied and a second input to which an adjusted reference voltage reduced by the reference resistance is applied. If the voltage difference between the measurement voltage and the reference voltage exceeds a preset value, a relay is switched on and a timing motor started. The difference in resistance is used to directly or indirectly generate a signal for optical and/or acoustic indications and/or trigger a regeneration process.
The actual application of the foregoing method was found to have serious drawbacks. In a series of tests, it was found that the resistances in the filter medium, for example, determined in two planes and used for subtraction, may differ greatly even after it has undergone uniform chemical treatment in those planes by uniform regeneration. Such deviations in the resistance behavior are serious because they can affect the subtraction process to such an extent that a definite detection of the load condition is no longer possible and/or faulty initiation of regeneration results.
A particular source of uncertainty in the foregoing method is that the resistances in the planes of measurement may be random or may deviate in the same or opposite directions, and the resulting errors in the subtraction process are further increased.
In the event relatively little regenerant is used during regeneration, as is presently called for in view of environmental pollution from widespread use of water softeners, the resistance difference needed for reliable signal generation is no longer available or else is within the error range of resistance deviations.
Another major drawback to the aforementioned known measuring process is that no distinction can be made between the regenerated and the loaded state of the exchanger filter layer. Partial loading states also cannot be distinguished when the filter layer has been blended so that concentration gradients no longer exist in the flow direction. This happens for example, after regeneration is performed and the regenerant feed has failed. A filter controlled by the aforementioned prior art system would start operation again after faulty regeneration.
After prolonged operation of ordinary filter systems, the capacity of the ion-exchanger can be reduced for example by irreversible absorption of iron and/or manganese. A specific drawback of the known process is that unless a certain capacity of the ion-exchanger has been reached, regeneration is no longer initiated and the filter controlling process does not deliver softened water as a product. Soft water consumer systems connected to the outputs of such filter systems risk being destroyed as a result of such behavior of heretofore known measuring processes.
It is therefore an objective of the present invention to cope with the problems of determining the degree of loading of ion-exchangers and to perform such determination with respect to both layered and unlayered exchangers as well as to determine the efficiency of regeneration and perform a measurement operation that requires apparatus that is simple to manufacture, easy to install in filter systems, largely trouble-free, capable of reliably controlling a filter system and preventing the discharge of untreated raw water.
Further objects of the invention include relativized measurement of the ion mobility with respect to a saturation concentration, preferably in the loaded ion-exchanger state and use of signals developed therefrom for controlling the filter systems, particularly water softener filters.
Still further objects of the invention include higher reliability of operation and reduced regenerant consumption for less environmental pollution.